Severity of OSA Linked to Sedentary Lifestyle

January 24, 2009 -- Not only is OSA linked to insulin resistance and liver disease independent of
obesity, but at least one risk factor is also common to obesity and OSA:
prolonged daytime sitting or standing. Even when the sedentary lifestyle does
not lead to obesity, it may still lead to OSA and its concomitant health risks,
according to another research article in the first issue for February of the
American Journal of Respiratory and Critical Care Medicine.

“Overnight
fluid displacement from legs, related to prolonged sitting, may play a
previously unrecognized role in the pathogenesis of OSA,” wrote principle
investigator, T. Douglass Bradley, M.D., professor of medicine and director of
the Centre for Sleep Medicine and Circadian Biology at the University of
Toronto,

The research also found that the volume of fluid shift was
directly linked to the hours in a day that the subject reported sitting or
standing and was independent of the excess weight that often accompanies
sedentary lifestyles.

“In more recent years, the introduction of modern
technologies into the workplace has greatly reduced the need for physical
activity and increased the number of jobs requiring prolonged sitting, during
which absence of the contraction of calf muscles leads to dependent fluid
accumulation in the legs that is proportional to the time spent in that
position,” explained Dr. Bradley. “When assuming the recumbent position at
bedtime, the fluid retained in the legs during the day in redistributed to the
upper body. It is therefore plausible that some of the displaced fluid might
reach the neck and predispose to upper airway constriction.”

To
determine whether that, in fact, was the case, the researchers recruited 23
nonobese subjects who were being evaluated for suspected OSA and performed
standard sleep studies that assessed each subject for sleep stages and number of
arousals, as well as oxygen saturation of the blood. The circumferences of their
calves and necks were also measured at bedtime and upon awakening, before they
got up.

Indeed, they found that the only significant correlate factor
with respect to severity of OSA was the overnight change of fluid volume in the
leg, which explained 67% of the variance in OSA severity. The change in fluid in
the leg was, in turn, directly related to the amount of time the subject
reported sitting each day.

“An important implication of our observations
is that sedentary living may predispose to OSA not only by promoting obesity,
but also by causing dependent fluid accumulation in the legs, which can shift
rostrally to the neck overnight,” said Dr. Bradley.

This finding may also
help explain why 40 percent of patients with OSA are not obese, and why a
reduction in OSA has been described when subjects begin exercise programs, even
in the absence of weight loss.

A news release courtesy of American Thoracic Society, a leading medical association dedicated to advancing pulmonary, critical care and
sleep medicine.